1. Field of the Invention
The present invention relates to a simulcast technique which can prevent the reception ratio from being lowered due to a phase difference of signals generated by data transmission time differences from a base station at a signal overlapping area between base stations in a paging system having a plurality of base stations.
2. Description of the Related Art
Generally, in an earlier paging system, as shown in FIG. 1, page data is transmitted from a paging system 10 to transmitters 24 and 26 of base stations 20 and 22 connected via transmission lines 12 and 14 and transmission equipment 16 and 18, time delays between the transmission lines 12 and 14 and transmission equipment are generated. The deviation in the time delay depends on the characteristics of various respective equipment, and the time delay becomes different depending on the number of transmission equipment and the length of the transmission line. Because of the time delay, a plurality of transmitters 24 and 26, contained within a plurality of base stations, have a time deviation for each transmission time so as to generate the overlapping area of signals generated by the transmitters 24 and 26 of the respective base stations 20 and 22, as shown in FIG. 2. The signals received from the overlapping area 28 exhibit a phase difference between two signals received from a first base station and a second base station, as shown in FIG. 3. A signal distortion is generated at the overlapping area of two signals different in their phases, thereby lowering the reception ratio of a pager receiver 25.
An earlier technique for solving the problem that the reception rate is lowered at the overlapping area will now be described with reference to FIG. 4. First, a main transmission line 12 for transmitting page data and first and second reserve lines 32 and 34 are connected between a line delay time measuring device 30 installed within a paging system 10 and a line switching unit 46 installed within a first base station 20. The line switching unit 46 is controlled by a base station controller 44 installed in the first base station 20. The line delay time measuring device 30 transmits a command to the base station controller 44 so that loops between the main transmission line 12 and first reserve line 32 and between the main transmission line 12 and second reserve line 32 are connected, and then transmits predetermined data to measure each loop delay time. A method of calculating the measured loop delay time is explained as follows. Assuming that the loop delay time T.sub.XY between the main transmission line 12 and first reserve line 32 is 2000 .mu.sec, the loop delay time T.sub.XZ between the main transmission line 12 and second reserve line 34 is 2500 .mu.sec, and the loop delay time T.sub.YZ between the first reserve line 32 and second reserve line 34 is 2200 .mu.sec, the delay time T.sub.X of the main transmission line 12 will be calculated as follows. EQU T.sub.XY =T.sub.X +T.sub.Y ( 1) EQU T.sub.XZ =T.sub.X +T.sub.Z ( 2) EQU T.sub.YZ =T.sub.Y +T.sub.Z ( 3)
Now, T.sub.X is obtained by manipulating the above noted simultaneous equations. ##EQU1##
It has been my observation that these types of systems inherently delay transmission of data. By way of explanation, T.sub.X is the delay time from the paging system 10 of the main transmission line 12 to the first base station 20, T.sub.Y is the delay time from the paging system 10 of the first reserve line 32 to the first base station 20, and T.sub.Z is the delay time from the paging system 10 of the second reserve line 34 to the first base station 20. As described above, the respective delay times from the main transmission line 12 to the plurality of base stations 20 are measured and then a reference value is set. Since the remainder obtained by subtracting the respective delay times from the reference value is the time delayed by the transmitters, all of the base stations can simultaneously transmit data. Therefore, in the case of three base stations, it is assumed that the delay time from the paging system 10 to a first base station is 2500 .mu.sec, the delay time from the paging system to a second base station is 3000 .mu.sec, the delay time from the paging system to a third first base station is 3500 .mu.sec, and the reference value is set to 5000 .mu.sec, the delay time of the transmitter of the first base station becomes 5000-2500=2000(.mu.sec),the delay time of the transmitter of the second base station becomes 5000-3000=2000(.mu.sec), and the delay time of the transmitter of the third base station becomes 5000-3500=1500(.mu.sec). In accordance my observation, it is demonstrated therefore, that since the data is delayed constantly, i.e., by 5000.mu.sec, to be transmitted from the paging system, irrespective of the delays of the transmission lines, the transmitters of the base stations can transmit data simultaneously.
I have found however, that according to such technology, a separate line delay time measuring device must be provided within the paging system to calculate the loss generated between the respective lines and transmitters. Also, a predetermined reference value is set and a delay time given in transmitting page data corresponding to the reference value. Thus, complex operations for calculating every transmission loss must be performed.
I have also found that, when the page data is transmitted from the paging system to base stations, the transmission loss is different for respective base stations. Thus, even if delay time is given using reserve lines, based on a reference loss, the transmission delay is not constant.
The following patents each disclose adjustment techniques for simulcast arrangements in paging systems but do not teach or suggest the specifically recited features of the present invention.
U.S. Pat. No. 5,416,808 to Witsaman, et al. entitled Apparatus For Synchronizing A Plurality Of Clocks In A Simulcast Network To A Reference Clock. PA1 U.S. Pat. No. 5,481,258 to Fawcett, et al. Entitled Method And Apparatus For Coordinating Clocks In A Simulcast Network. PA1 U.S. Pat. No. 5,561,701 to Ichikawa entitled Radio Paging System Having A Plurality Of Transmitter Stations. PA1 U.S. Pat. No. 5,239,671 to Linquist, et al. entitled Simulcast Satellite Paging System With Provision For Signal Interruption. PA1 U.S. Pat. No. 5,483,665 to Linquist, et al. entitled Simulcast Satellite paging System With Over Lapping Paging Reception Locales.